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EVAPORATION .                                    367
Recently another system of evaporation has been developed in Europe, in which vapors coming from the evaporator are compressed in a multi-stage turbo-compressor, and then returned into the steam chest at higher temperature. It is claimed that under certain conditions, the fuel consumption of a single effect is less than that of a quadruple effect. For detailed information, see an article by Carlsson in Chemical and Metallurgical Engineering, Apr. 13, 1921, p. 645.
Water Consumption.—With few exceptions, the vapors coining from the last effect must be condensed in some kind of a condenser, and the amount of water needed for this purpose is frequently a very important factor.
The quantity of cooling water can be determined by the following equation:
W = Vi X L/(ti -10 -t) where,
W denotes the quantity of cooling water in pounds, Vi denotes the amount of vapor coming from the last effect, L denotes the latent heat of this vapor, ti denotes the temperature of this vapor, t denotes the temperature of the cooling water entering the cc arid
ti — 10 denotes the temperature of the water discharged from the cc Well constructed condensers will operate satisfactorily with a different between the temperature of the vapor and the hot water; but it is safer to fij difference of 10° for actual operating conditions, and therefore this figure has I in the above equation.    The temperature t is known, and ti and L can be figu the vacuum at which the last effect is supposed to work (see steam tables in J.VCJJLU ur Marks, also a short table, p.. 48, this book).    Vi can be determined from the data given on page 365, covering factors of distribution.
Evaporation and Percentage of Solids.—In many cases only the amount of weak or heavy liquor and the specific gravity or percentage of solids of solutions are known. The determination of the steam and water consumption is, however, based on the actual amount of water evaporated, and this quantity may be figured as follows:
V = L(l - l/h) = H(h/l - 1)
where V denotes total quantity of water evaporated in pounds,
L denotes amount of weak liquor in pounds,
H denotes amount of heavy liquor in pounds,
Z denotes percentage of solids in weak liquor or its specific gravity minus 1, and
h denotes percentage of solids in heavy liquor or its specific gravity minus
1.
These equations may also be used to figure out various factors useful for evaporator practice:
H = L X l/h; L = H X h/l] I = h(l - V/H)
H = V/(h/l- 1);L = V/(l - l/h)i h = Z/(l - V/L)
The factors of distribution mentioned on page 365, and the above formula will give the data to figure the percentage of solids or specific gravity of the solution in each effect of a multiple-effect evaporator. This is frequently important for the design and operation of evaporators, as it gives means for the determination of the boiling point, viscosity and crystallizing point.